mm/highmem: add notes about conversions from kmap{,_atomic}()

kmap() and kmap_atomic() have been deprecated.  kmap_local_page() should
always be used in new code and the call sites of the two deprecated
functions should be converted.  This latter task can lead to errors if it
is not carried out with the necessary attention to the context around and
between the maps and unmaps.

Therefore, add further information to the Highmem's documentation for the
purpose to make it clearer that (1) kmap() and kmap_atomic() must not any
longer be called in new code and (2) developers doing conversions from
kmap() amd kmap_atomic() are expected to take care of the context around
and between the maps and unmaps, in order to not break the code.

Relevant parts of this patch have been taken from messages exchanged
privately with Ira Weiny (thanks!).

[[email protected]: merge two sentences into one, per Bagas]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Fabio M. De Francesco <[email protected]>
Cc: Ira Weiny <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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